A magnitude 4.2 earthquake has shaken part of Northern California between the state capital and the San Francisco Bay region, setting off automated quake alerts. The U.S. Geological Survey says the 9:29 a.m. Wednesday quake was centered in the agricultural Isleton area about 32 miles southwest of Sacramento. The area is largely occupied by the Sacramento-San Joaquin River Delta. The tremor triggered a warning from the West Coast’s ShakeAlert system, which detects the start of a quake and sends alerts to smartphones in areas expected to experience shaking. In Sacramento, a ShakeAlert arrived before the shaking started. The quake initially was estimated at magnitude 5.7 but was downgraded.